put it on a 1:2 or 1:3 ratio. The big magmotor is pointless as you will waste so much weight in the motor. Oh and to handle the torque you will need to make sure that you have heavy duty belts or chains to handle it

Mind you back in the day I had hornet 2 on a 1.8kg disc with the mini mag. Worked fine on a 1.5:1 ratio.

For a brushed motor I'd just go with a big solid state relay. Don't bother with a controller as it will fry unless severly oversized.